Recently, there is an increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention.
Among secondary batteries which are now in use, lithium secondary batteries developed in the early 1990s are in the spotlight due to the advantages of higher drive voltages and far greater energy densities than those of conventional batteries using an aqueous electrolyte, such as Ni—MH, Ni—Cd and H2SO4—Pb batteries.
A secondary battery includes a cathode, an anode, a separator, and an electrolyte comprising an electrolyte solvent and an electrolyte salt. Meanwhile, a conventional secondary battery comprising an anode formed of a carbonaceous material and a cathode formed of a lithium metal oxide has an average discharge voltage of 3.6˜3.7V. To obtain such a drive voltage, it is necessary to provide an electrolyte composition that is stable in a charge/discharge voltage range of the battery, for example, in a range of 0 to 4.2V.
However, currently used electrolyte solvents have problems in that they are decomposed on the surface of an electrode during charge/discharge cycles of a battery, and are co-intercalated into a gap between carbonaceous anode active material layers to cause a structural collapse of the anode, resulting in degradation of the stability of the battery. Meanwhile, it has been known that the above problems could be solved by a solid electrolyte interface (SEI) film formed on the surface of an anode via the reduction of the electrolyte solvent upon the first charge cycle of the battery. However, the SEI film is not sufficient to serve as a lasting protective film for the anode. Therefore, the above problems still remain unsolved during repeated charge/discharge cycles, and may cause a drop in lifespan characteristics of the battery. Particularly, the SEI film is not thermally stable, and thus may be easily broken down by electrochemical energy and heat energy increasing with the lapse of time when the battery is driven or stored at high temperature. Accordingly, gases including CO2 are generated continuously due to the collapse of the SEI film, decomposition of the electrolyte, etc., under such high temperature conditions, resulting in an increase in the internal pressure and thickness of the battery.
To solve the aforementioned problems, a method using vinylene carbonate (referred to also as VC hereinafter) as an electrolyte additive capable of forming a SEI film on an anode is suggested. However, the SEI film formed by VC is decomposed with ease when exposed under high temperature conditions, resulting in degradation of the high temperature stability of a battery.